The present invention relates to hard disk drives used to store data, and more particularly to a head-media system having reduced stiction and low fly height capability.
In the field of hard disk storage systems, continuous improvements have been made in increasing the area density, i.e., the number of stored bits per unit of surface area. As is well known, decreasing the fly height of the read/write head results in reduced pulse width (PW50) due to a number of factors which allows for greater recording density. For a discussion of the effects of lower fly height, see, for example, U.S. Pat. No. 5,673,156. In any event, bringing the head closer to the media has been a key area of effort in increasing recording densities.
The read/write head is typically a part of or affixed to a larger body that flies over the disk and is typically referred to as a xe2x80x9csliderxe2x80x9d. The slider has a lower surface referred to as the air bearing surface. The air bearing surface typically comprises one or more rails which generally generate a positive air pressure. In addition, there is often a cavity or similar structure that creates a sub-ambient pressure to counterbalance the positive pressure to some extent. The slider body is attached to a suspension via a head gimbal assembly which biases the slider body towards the disk. The net effect of the air bearing surface and the suspension is to cause the slider to fly at the desired height when the disk is at full speed, and to cause the slider to be in contact with the disk surface when the disk is at rest. The portion of the slider that contacts the disk is typically the aforementioned one or more rails. As the fly height of the slider is reduced, it is necessary to produce disks with increasingly smooth surfaces. As is well known, the slider undergoes sliding contact with a portion of the disk whenever the drive motor is turned on or off. This contact between the slider and the disk occurring when the drive is turned on and off is known as contact start stop (CSS) operation.
The CSS motion between the slider and the disk is of great concern in the reliability of the drive since it is generally the major initiator of failure in hard disk drives. In today""s commercially available disk drives, generally 20,000 CSS cycles for desk-top computer applications and up to 100,000 CSS cycles for portable or hand-held computer applications is considered adequate. A greater number of CSS cycles is needed in portable and hand-held computer applications because the drives are frequently turned on and off to conserve battery power. Recently, there has been a trend to reduce power consumption in desktop computers. Therefore it is expected that CSS requirements will greatly increase for desktop applications as well.
In order to improve the CSS performance, it is well understood that friction must be minimized between the slider and the disk. Static friction or stiction is a term used to describe the force exerted against the motion of the slider relative to the disk surface when the slider is at rest on the disk surface. Stiction values are often given in grams to represent the force required to separate the slider from the disk. The stiction is greatly increased if the lubricant that is used on the surface of most disks wets a significant portion of the slider/disk interface.
Often, the term initial stiction refers to the stiction encountered when the slider contacts the disk for a minimal amount of time, without a significant opportunity for lubricant to migrate to the slider/disk interface. Parking stiction is a term used when the disk drive has not been in use, so that the slider has been at rest on the CSS zone for some time and may have some lubricant migration to the interface. Parking stiction is typically greater than initial stiction. Finally, the term fly stiction is used to describe the situation where the slider has flown over the disk for a considerable amount of time so as to pick up lubricant, and then after returning to the disk surface has remained on the disk surface for a sufficient time to allow the lubricant to flow to and significantly wet the interface, thereby greatly increasing stiction. Stiction can be strong enough to prevent the drive motor from turning, or worse yet, can damage the head, cause the slider to become detached from the suspension assembly, or cause the slider to ding the disk surface during separation of the slider from the disk surface. (The term xe2x80x9cdingxe2x80x9d is used in the art to describe an abnormal and sudden impact of the slider against the disk surface which dents the disk surface around the impact area. This can occur, for example, by accidentally dropping the disk drive on a hard surface. This can also occur when the slider is stuck on the disk surface during drive start-up due to high stiction, followed by sudden release of the slider, which causes it to bounce on and thereby dent the disk surface.)
It has been recognized that stiction can be reduced by putting a xe2x80x9cmicro-texturexe2x80x9d on the disk surface to reduce the effective contact area between the slider and the disk. See, for example, Marchon et al., xe2x80x9cSignificance of Surface Roughness Measurements. Application to the Tribology of the Head/Disk Interface,xe2x80x9d Tribology and Mechanics of Magnetic Storage Systems VI, ASLE SP-26, page 71 (1990), which describes the roughness needed to achieve an acceptable rate of increase in stiction under prolonged CSS for a disk comprising an aluminum/NiP substrate with a near concentric texture pattern. Also, Lee et al., describe the effect of texture crossing angle on CSS performance in xe2x80x9cEffect of Disk Cross Hatch Texture on Tribological Performancexe2x80x9d, published in IEEE Transaction on Magnetics, Vol. 28, No. 5, September 1992, pp. 2880-2882. In effect, a rougher texture and modification of texture morphology is needed to achieve acceptable CSS performance. The texture pattern may be put on the disk by mechanically abrading the substrate surface using well known methods.
In contrast to the requirements of CSS operation, for reading or writing data it is desirable that the surface of the disk be as smooth as possible to allow the head to fly as close as possible to the disk surface. Because of these differing requirements, it is known to use zone texturing where a portion of the disk used for CSS operation (the CSS zone) is textured more heavily than the portion of the disk used for data storage (the data zone). One problem with such zone texturing, however, is that it is difficult to create a precisely delineated CSS zone with mechanical texturing methods. Because of this, some portion of the data zone is typically lost, thus reducing the amount of data a disk can hold.
Because the data zone is smoother than the CSS zone, both the glide height (minimum distance at which a slider may fly without contacting any portion of the disk surface) and the glide avalanche height (distance above mean disk surface level at which the slider makes regular and continuous contact with the disk surface) are lower in the data zone than in the CSS zone. However, because it is necessary to move the head from over the data zone to the CSS zone, the glide avalanche height of the CSS zone limits the fly height over the data zone, as the head must be able to safely move between the two zones, without undue contact in the CSS zone which could lead to wear of the disk surface, the slider, and generation of debris. It should be noted that it is difficult to produce mechanical texturing with a high degree of uniformity. This nonuniformity in surface texture means that some portions of the CSS zone may be considerably rougher than average, which poses further limitations on the fly height.
Another known method to provide the necessary texture in the CSS zone is laser zone texturing. An example of this method is described in U.S. Pat. No. 5,108,781. In such a method, a laser beam is focused to a small spot on the disk surface, forming uniformly shaped and sized features in a controllable pattern. Because of the high degree of control possible with a laser system, the CSS zone can be precisely delineated so that loss of data zone area can be minimized. Furthermore, because the size of the features is better controlled than the surface morphology resulting from mechanical texturing, the above-described uniformity problem is greatly reduced. However, because the surface in the laser texture zone has a considerably greater roughness than the data zone, the CSS zone still provides a limitation to the fly height even in laser zone textured disks. See xe2x80x9cThe Special Needs of Server Class Drivesxe2x80x9d by Wachenschwanz et al., IDEMA Insight, Vol. XI, No. 1, January/February 1998 which illustrates that laser zone texturing achieves acceptable stiction performance for today""s devices and further asserts that laser based zone textured disks should be extendible for at least two generations.
Another method to reduce stiction in CSS operation is to provide a texture on the surface of the slider rather than the disk. Such sliders are frequently referred to as xe2x80x9cpaddedxe2x80x9d sliders or xe2x80x9cstiction-freexe2x80x9d sliders. The texture may be provided in a variety of manners. For example, xe2x80x9cNumerical Simulation of the Steady State Flying Characteristics of a Fifty Percent Slider with Surface Texturexe2x80x9d by Wahl et al., IEEE Transactions on Magnetics, Vol. 30, No. 6, November 1994, discloses a slider having a plurality of hemispherical, conical, or cylindrical features arranged in a densely packed pattern thereon. U.S. Pat. No. 5,079,657 teaches several varieties of textured sliders using chemical etching in one embodiment formed by differential etching, and in another embodiment formed by the use of a masked photo resist layer. xe2x80x9cStiction Free Slider for Lightly Textured Disksxe2x80x9d, by D. Yamamoto et al., IEEE Trans. Mag. Vol. 34, No. 4, 1998, shows a textured slider which has one or more xe2x80x9cpadsxe2x80x9d along the length of each rail. Herein, a slider having texture formed by any method, including the foregoing, with any type of pattern is referred to as a textured slider.
FIGS. 1A and 1B show two examples of textured sliders. As shown in FIGS. 1A and 1B, the sliders comprise a slider body 101a/b coupled to suspension 102a/b. Each of the sliders comprises two rails 103a/b (although sliders with a single rail and sliders with more than two rails may be used). Also as shown in FIGS. 1A and 1B, each of the rails has a plurality of pads 104a/b. In the particular slider shown in FIG. 1B, each pad 104b may have dimensions, for example, of approximately 35-50 microns wide by 50-100 microns long. Of course other dimensions may be used.
In the above described textured sliders, the intent is to provide a slider surface that has some portions at a different elevation than others to reduce the total contact area and thereby reduce stiction. One advantage to using such sliders is that a lower roughness of the disk surface is needed to meet stiction requirements. This lower roughness is comparable to the roughness of current data zone texture, so that the entire disk surface may be textured as appropriate for data storage, thus allowing for lower fly heights and increased density. Additionally, textured sliders are intended to eliminate the need for a separate zone, whether by mechanical texturing with its concomitant loss in usable area, or laser zone texturing which typically adds a step to the disk fabrication process. In the above mentioned article by Yamamoto et al., it is stated that the stiction results obtained with the stiction free slider described therein is acceptable even on relatively lightly textured surfaces which have a roughness comparable to current data zone texture. Recently, it has been reported that a textured slider may be extendible for the next several generations of disk drives. See xe2x80x9cFujitsu""s Padded Slider Hold Stiction at Bayxe2x80x9d, Data Storage, May 1998, page 8.
A further approach to the stiction problem is drives using a so-called xe2x80x9cload/unloadxe2x80x9d mechanism. In these drives, when the drive is turned off, the head is parked on a ramp and not on the disk surface. Therefore, in load/unload drives, the problem of stiction is eliminated. However, the load/unload mechanism adds to the cost and complexity of the drive.
As can be seen from the foregoing, current attempts are to either improve the disk texturing, with particular current emphasis on laser zone texturing or alternatively to eliminate the need for a separate zone by providing a textured slider or by providing a load/unload mechanism.
As recording density increases, ever smoother surfaces will be required so that heads may fly lower. Current state-of-the-art systems have glide avalanche heights in the data zone of approximately 0.8 through 1.0 microinch (xcexcxe2x80x3). In the future, glide avalanche heights of approximately 0.4 xcexcxe2x80x3 or below will be needed for disks having densities in the range of approximately 3-5 gigabits per square inch (Gb/in2). On a laser zone textured disk, the glide avalanche height for such CSS zone would need to be in the range of approximately 0.6-0.7 xcexcxe2x80x3. An average laser bump height in the range of approximately 50-100 angstroms (xc3x85) will provide a glide avalanche height in this range, but is likely to have unacceptably high stiction for conventional sliders. Thus, what is needed is a method and apparatus for providing a slider-head system having very low glide height and acceptable stiction performance.
An embodiment of the present invention comprises a slider and a disk for storing magnetic data. The slider is textured to reduce stiction. The disk has a contact area for CSS operation (CSS zone) having a surface roughness greater than that of an area for data storage. The surface roughness may be created by any means such as mechanical texturing, top surface texturing, sputter texturing, or may comprise a pattern of features with relatively uniform height such as those formed by laser texturing. The CSS zone can be made with a sufficiently low average height to allow for the low fly heights of advanced disk drives. By use of a textured slider and a disk having, for example, laser features in the CSS zone, stiction is kept sufficiently low to allow for reliable operation.
Other features and advantages of the present invention will become apparent from the detailed description, figures and claims which follow.